This invention relates in general to the manufacture of color television picture tubes and in particular to an apparatus for suppressing erosion of the electron emissive coating on the cathode of an electron gun during manufacture of the tube. Conventionally, an electron gun used in a color television picture tube includes an electron beam source and an electron beam focus lens. The electron beam source typically comprises a heated cathode element and associated electrodes which collect electrons emitted by the cathode element and form them into a beam cross-over. The electron beam focus lens shapes the stream of electrons emitted by the cathode and focuses the beam cross-over on the screen of the tube. The electron beam focus lens typically comprises electrodes at varying potentials. The forward element is the focus lens anode and typically takes the form of a cup called a "convergence" or "shield" cup.
An electron gun for use in a color television picture tube generally comprises three guns, one each for exciting red, blue and gree phosphor elements on the screen of the tube. Each of the electrodes and the shield cup in the gun have three apertures, one for each of the three cathodes which emit the streams of electrons. The apertures are generally circular and the apertures for each beam lie on a common line, that is they are coaxial. The apertures in the electron gun form beam passageways.
In the manufacture of color television picture tubes or black and white tubes, after the tube is assembled, most of the gas, usually air, which is inside the tube must be evacuated. Conventionally, this is done by attaching a vacuum pump to a tubulator which is located at the rear of the neck of the tube. As the tube is evacuated, all of the gas which is drawn from the tube must move through the neck of the tube and thus through the electron gun situated in the tube neck. The beam passageways through the gun unavoidably act as high velocity gas conduits as the tube is evacuated. These high velocity gas conduits create a violent flow of gas over the cathodes while the tube is being evacuated. It has been observed that this violent flow of gas over the cathode causes erosion of the electron emissive coating on the cathode (especially the coating of the "green" cathode in an electron gun for a color television picture tube) which may necessitate rejection of a tube or which may result in degraded performance and/or reliability of a tube.
It is common practice in the manufacture of television picture tubes to control the humidity during evacuation of the tube within a narrow dew point window. Typically the dew point is controlled between 40.degree. F. to 50.degree. F. If the humidity is too high the cathode coating is eroded by particles of moisture during evacuation. This, coupled with the high velocity at the initiation of evacuation and the violent flow of the air through the electron gun in the neck of the tube results in serious erosion of the cathode coating. The narrow dew point window has always presented serious problems in the manufacture of color television picture tubes. The conventional factory process must be constantly and closely monitored and the dew point window shifted with the seasons of the year. The present process is so difficult as to be barely workable. Suppression of the cathode erosion would allow the dew point window to be opened up, and thus allow the same process to be used year around.
Copending application Ser. No. 782,140 now U.S. Pat. No. 4,132,459 discloses a method of preventing cathode coating erosion wherein gas is pumped slowly from the tube so that the flow of gas through the electron gun does not occur at such a high reate as to cause erosion of the cathode coatings. This method has several drawbacks: (1) an undesirably lone time is needed to evacuate the tube, and (2) also the method is not totally reliable.
Copending application Ser. No. 784,478 now U.S. Pat. No. 4,137,480 discloses a unique electron gun having at least one cathode having an electron emissive coating, a forward element and a plurality of electrodes interspaced between the cathode and the forward element. The electrodes and forward element each have at least one aperture wherein the apertures in the electrodes and forward element are coaxial and define at least one beam passageway for passing through the gun a stream of electrons emitted by the cathode during operation. The beam passageway unavoidably forms a conduit for high velocity gas when the gun is located in a narrow neck of a television picture tube and the tube is evacuated of gas through a tubulator located at the rear end of the neck. The improvement in the electron gun comprises a gas influencing element for perturbing the high velocity as flow in the conduit at least in the region of the cathode as the tube is evacuated to suppress erosion of the cathode coating by preventing a violent flow of gas over the cathode.
British Pat. No. 1474-714 discloses the use of a protective coating over the electron emissive coating on the cathode for protection against water droplets during evacuation of the tube. After the tube is evacuated the protective coating is heated and an activated oxide cathode results.
This invention has general applicability and may be applied to electron gun assemblies in color television picture tubes as well as to electron guns in black and white tubes. The invention is known to have applicability to a television picture tube having a narrow neck utilizing either a standard type electron gun or a unique type of electron gun disclosed in U.S. Pat. No. 3,995,194, assigned to the assignee of this invention.